The displacement current is only marginally involved in the
interpretation of the (presumably electrostatic,) experiment.
But thermodynamic, mentioned by Dave Bowman, and chemistry,
may be very important. This calls for a chemical experiment;
I wish I had time to conduct it.
Take three "objects", plate of Zn, plate of Cu and an electrolyte
in a dish. Each object is initially neutral. We know that "neutral"
does not mean the absence of charges, only a perfect balance
between + and -. Insert the plates into the electrolyte and wait
till the constant d.o.p. is established. We know that at this stage
Cu is positive and Zn is negative. And we believe that +Q and
-Q are numerically equal while the electrolyte is neutral. I never
heard about experiments demonstrating this.
To conduct such experiment I would use Zn and Cu electrodes
which are geometrically identical and have bent sat the top to
create supporting platforms. Without removing the electrodes I
would place another Zn sheet on the Zn platform and another Cu
sheet on the Cu platform. These dry pieces are also geometrically
identical. Then I would remove dry pieces from their platforms
"simultaneously" and measure their charges with an electrometer.
Are the removed charges identical or different?
Yes, "simultaneously" is easier to say that to implement. To
overcome this I would make two measurements. One when Cu is
removed before Zn and one when Zn is removed before Cu. If
the electric charges on electrodes are numerically equal then the
sum of Q-Cu-1 and Q-Cu-2 should be the same as the sum of
Q-Zn-1 and Q-Zn-2. If the sums are not equal then the equality
of charges on the electrodes is questionable.
Some chemists on this lists do have what is necessary for the
experiment. It would be nice if somebody did the experiment
(or assigned it to a student) and described the outcome here.